Metal sections are produced particularly from non-ferrous metals, such as brass, copper etc., in extrusion presses which comprise a container forming a receptacle which is closed at one end by means of a die and in which is placed a piece of metal, called a billet, which is made to pass through the die by extrusion. The press must be fed with billets of a length, depending on the dimensions of the container and on the length of the section to be produced.
The billets feeding the press must therefore be cut from metal bars, which have a cross-section identical to that of the container and a length of several meters.
The press is therefore fed by a billet preparation installation which comprises a storage magazine for the bars, a device for conveying the bars in succession along a generally straight path, shears located at the exit of the conveying device, and a loading device, for example a pivoting arm which receives each billet at the exit of the shears and which places it in the axis of the container where it is introduced by sliding.
For some time, hot shearing has become increasingly common. The installation then includes a heating furnace of tubular shape located in the path of the bars. Thus, a conventional installation comprises a storage table, a conveying device equipped with a pusher pushing the bars onto a straight conveyor which passes through a tubular furnace, and shears located at the exit of the furnace and associated with a loading device.
So that the bar does not have time to cool before it is sheared, the shears are located as close as possible to the exit door of the furnace, and according to an arrangement which is the subject of applicants' French Patent No. 84/18811 the shears can even be attached to the furnace, the exit door of which is formed by one of the jaws equipped, for this purpose, with a refractory protective covering.
After shearing, the bar is retracted into the furnace in order to clear the door and allow the latter to be closed.
The length of the furnace depends on its mode of heating, and induction furnaces can be shorter. In all cases, however, the conveyor supports several bars placed in sequence. When the rear end of the last bar is sufficiently advanced, a stop in the advance, for example for shearing, is utilized order to retract the pusher by the length necessary for introducing a new bar coming from the magazine onto the conveyor; the pusher subsequently moves forward again in order to bring the new bar into contact with the preceding one and push the set along. Thus, a continuous set of bars in contact one behind the other passes through the furnace.
By causing this set to advance progressively in this way, a certain number of billets can be cut from the first bar in the line, but the length L of the bar does not correspond exactly to an integral multiple of the length b of the billet, the more so because the latter may vary according to market requirements. As a result, at the end of shearing of the bar, it is exceptional, unless appropriate arrangements are made, if the shearing plane coincides exactly with the parting plane between the bar at the end of cutting and the succeeding bar. The last billet is thus formed from two adjacent pieces, which are, respectively, the rear end of the bar being cut and the front end of the succeeding bar.
The fact that the billet is in two pieces is not a disadvantage if these are of suitable length. In contrast, when the ratio of the length of the piece to its diameter is below a certain limit, such a piece, called an "oddment", can no longer be handled without the risk of coming askew in the loading device or in the container.
The "oddments" of a length below a given limit, for example half the diameter, which, depending on the position of the shearing plane in relation to the parting plane, can be on one side of the latter or the other, have to be eliminated, and several arrangements can be employed for this purpose.
It is possible, first of all, to cut the billet to the normal length and, if an "oddment" occurs, eliminate this by hand. However, there is a risk that the cut will not be made under good conditions, and it becomes necessary to extrude a billet which is shorter than normal. Furthermore, eliminating the "oddments" by hand impedes the shearing cycle and, to avoid a loss of metal, makes it necessary for them to be recycled. It is therefore preferable as far as possible to avoid the actual elimination of the "oddments" and, on the contrary, act on the length of the sheared billets, so as to prevent remnants which are too short from occurring.
In a known arrangement, for example, the length of each bar is measured at the start of conveyance, the length of the remnant to be expected is calculated in advance as a function of the normal length of the billet, and, if this remnant is too short, its length is distributed over the various billets by changing the lengths of these accordingly, in such a way that the shearing plane of the last billet coincides with the parting plane.
In another process, described in DE-A-No. 3,120,464, after the length of the bar has been measured, the length of the expected remnant is calculated in advance, and, if this is too short, before the shearing of the last billet the remnant is cut into two pieces of a length greater than the handling limit, one of these pieces being held in reserve and the other forming a billet with an additional piece taken from the succeeding bar, the piece held in reserve then returning to the advancing cycle in order, in turn, to form a billet together with a second additional piece.
However, in all cases, it is necessary to measure the length of the bar and deduce the length of the remnant from it after a whole number of billets of the desired length has been cut.
The length of the billet can be determined relatively easily, for example, by placing at the desired distance downstream of the shearing plane a stop which controls the halting of the advance and the operation of the shears (U.S. Pat. No. 4,559,854).
The length of the bar can be measured at the exit of the magazine and its arrival on the conveyor. In fact, the latter can be equipped with a retractable stop or with a stationary sensor which emits a signal at the moment when the front end of the bar passes level with it. To determine the length of the bar, it is sufficient to locate the position of the pusher abutting against its rear end at the moment when the signal is emitted by the stationary detector. However, this measurement is made under cold conditions upstream of the furnace, and it is therefore necessary to take into account the expansions and other secondary effects which can affect the length of the bar. The calculation means of automated installations can take these various parameters into account, but the result of this is some inaccuracy in the measurement.
There has also been a proposal to mark the parting plane, for example by bringing back into the furnace by means of the conveyor the bar which follows the bar being sheared, the rear end of the bar being cut then separating from the front end of the succeeding bar (U.S. Pat. No. 4,559,854). The conveyor can also be equipped with a sliding track having an articulated end, in order, at the exit of the furnace, to cause the bar being cut to be offset relative to the succeeding bar still in the furnace. Such devices make it possible to mark the parting plane and measure its position very accurately, but they are necessarily placed between the shears and the furnace and therefore require the shears to be moved away from the furnace exit, thus increasing the length of the bar which has to be reintroduced into the furnace after shearing, and consequently the time of the shearing cycle as well as the risk of cooling of the bar.